Whether or not he was the best, he was certainly the highest ranked as he held the number #1 ranking without ever winning a Slam. In contrast to the women's tour where there have been no less than 3 Slamless players to achieve the #1 ranking in recent years (Safina, Jankovic and Wozniacki) I think Rios was the only player on the men's tour to achieve the #1 ranking without ever winning a Slam, at least in the Open Era.